1. Field of the Invention
The present invention relates to a noise filter constructed of a nonolithic sintered body obtained by laminating a plurality of ceramic green sheets having varistor characteristics which separate a set of electrodes, followed by cofiring.
2. Description of the Prior Art
In recent years, the development of microcomputers has resulted in microcomputers being incorporated in various apparatus such as industrial equipment, household electrical appliances and communications equipment.
Equipment carrying a microcomputer for digital control processing is liable to be destroyed or erroneously operated due to noise. An accident can actually happen due to noise. In many cases, the path of entrance of noise into the equipment is a power supply portion and a signal interconnection portion. Consequently, the function of a noise filter used in an input/output portion is taken seriously.
In order to solve the problem of EMI noise, it is necessary that the noise does not go in and out of the equipment. The noise normally goes in and out of the equipment through an electric wire. Consequently, a method of connecting a noise filter to the electric wire in the vicinity of the input/output portion of the equipment has been most commonly used.
As noise filters conventionally used, the following noise filters have been known: (a) one using a capacitor, (b) the capacitor-inductor composite system, (c) one using an inductor, and (d) one using a varistor.
The noise filter using a capacitor (a) is superior in that it reduces very small noise but cannot absorb a high-voltage pulse such as a static charge by only the capacitor. Accordingly, the destruction and the maulfunction of an apparatus or equipment incorporating the noise filter based on high-voltage pulse noise cannot be prevented.
The noise filter of the capacitor-inductor composite system (d) is suitable for absorption of white noise or the like. In addition, the noise filter using a varistor (d) is suitable for absorption of transient noise.
However, absorption of noise on a signal line is closely related to the frequency of a signal. Further, since digitization has recently proceeded, large distortion must not occur in a waveform. Therefore, the noise filter cannot be used as a noise filter for a signal line if its capacitance is too large. In addition, the withstanding voltage of an IC is approximately 60 to 70 volts. Accordingly, a noise component must be suppressed to 60 to 70 volts or less.
The noise filter of the capacitor-inductor system has no problem in terms of its capacitance. However, there is a problem that the transient noise in a resonance frequency band is passed without any modification, thereby to make it impossible to protect a circuit. Furthermore, in the case of high-voltage noise, the waveform can be changed but its peak value cannot be greatly restrained.
In the noise filter using a varistor, on the other hand, if its capacitance is 500 pF or more and a signal of hundreds of kilohertzs to tens of megahertzs such as a signal which is passed on a signal line is handled, the signal per se may be absorbed or deformed. Presently, therefore, a varistor is not commonly used as a device for reducing noise on the signal line.